The complexity of mutational mechanisms operating in the bacterium E. coli is studied by DNA sequence analysis of large groups of lacI- mutants produced under selected conditions. LacI- mutations are selected on an F'prolac and then transferred by genetic (homologous) recombination onto a single-stranded phage vector for DNA sequencing. An analysis of the resultant mutational spectra has yielded clues with regard to the mechanisms of both spontaneous and SOS-mediated mutagenesis. Spontaneous mutation is a highly varied process in which multiple classes of mutations are produced. DNA replication errors on the other hand are of mainly two classes, base substitutions and single-base frame-shifts. Mismatch repair and the occurrence of mutations from sources other than DNA replication are responsible for this discrepancy. The mechanisms of SOS-induced mutation were investigated by studying the specificity of ultraviolet-light-induced mutation. It was shown that wild-type and excision-repair-deficient strains of E. coli exhibit a very similar specificity which has implications for the mutagenic pathways in these strains which have been thought to be distinct. The data could furthermore be used to draw conclusions regarding the nature of the premutagenic lesions in DNA after UV-irradiation.